Did you by any chance install ECK via Helm? If so you might be hitting an issue with the validating webhook where the webhook for the v1beta1 API version validates your request where the file realm support was not present yet.
A workaround is outlined here Elastic Search, not able to modify the settings - #5 by charith-elastic
We will also update our documentation to call out the issue and ship a fix for the Helm chart as soon as possible.